This invention relates to the measurement of a magnetic field, and, more particularly, to an apparatus and method for measuring the vector components and spatial variation of a magnetic field.
Many types of objects emit magnetic fields that can be measured externally to the object, using a device called a magnetometer. Magnetic measurements are noninvasive, permitting an understanding of the interior of the object without physical penetration of the object. In an example drawn from the field of biomagnetism, the human brain and heart produce magnetic fields that extend outside the body. The measurement of these fields provide an indication of the operation of the measured organ, without physical penetration or radiation of the body. In an example from another field, nondestructive evaluation, defects in objects such as pieces of metal can be located by measuring magnetic fields external to the object. Anomalies in the measured field may be traced to defects within the metal.
The magnitude of external magnetic fields produced by a body generally decrease rapidly with increasing distance from the body. Particularly if the magnetic field is initially small, it is important to place the magnetometer close to the surface of the body and/or to use a very sensitive magnetometer. The most sensitive magnetometers now available are those using a magnetic field sensor such as a wire loop that produces a small electrical current when magnetic flux penetrates the loop, and a Superconducting Quantum Interference Device (SQUID) that detects the small electrical current.
The application of magnetometer and magnetic sensing techniques to nondestructive evaluation and other analytical purpose is often limited by the available sensitivity and spatial resolution of the sensor systems. To permit the SQUID to operate, as well as to obtain the most sensitivity and highest signal-to-noise ratio, the magnetometer is cooled to superconducting temperatures, and typically to liquid helium temperature (4.2 K.). The magnetometer is placed into a cryogenic dewar to achieve the cooling. As a rule, the thickness of the dewar walls and the configuration of the dewar limit how closely the magntic field sensor may be placed to the surface of the object being measured. Consequently, the spatial resolution and sensitivity of the sensor/SQUID detector are limited as well.
There exists a need for an improved approach to measuring magnetic fields using sensors and SQUID detectors. The approach should be capable of measuring the vector components of the magnetic field and the spatial variation in those components. It should also permit the sensors to be placed very close to the surface of the object. The present invention fulfills this need, and further provides related advantages.